for wells whence men take the water required for domestic use; and it is 

 the reservoir which holds storm waters and equalizes the flow of brooks and 

 rivers. 4 



We may add that also it is the source of a large part of the 

 increasing quantity of water used by industry, and, perhaps 

 most important of all, that without this store to draw upon 

 the vegetation which provides food, clothing, and shelter 

 generally could not exist. 



"It appears", says Robert E. Horton 



that with extensive ground-water storage there is a very definite relation- 

 ship between ground-water level and stream flow in the absence of rain, 

 whereas in certain other classes of areas, owing either to a lack of per- 

 manent ground-water table or to complicated ground-water conditions, 

 there may be but little relationship between ground-water level and 

 stream flow. Areas where the relationship exists are, however, of wide 

 extent * * *. 5 



Most persons will perhaps be surprised to learn how large a portion of 

 the total run-off of streams in permeable soils, such as prevail in the Upper 

 Mississippi Valley, is derived from ground water. 6 



This is rarely less than 40 to 50 percent even for streams with shallow 

 soils, and may run close to 100 percent in the case of streams whose basins 

 are covered with deep, sandy, glacial deposits, such as occur in large areas 

 in Michigan, Wisconsin, and Minnesota. 6 



Natural absorption, infiltration, and maintenance of 

 underground storage are of critical importance. This is not 

 only because of the value of the underground store itself. If 

 the normal process of providing that store through absorption 

 and infiltration is not maintained, excessive run-off will result, 

 and this in itself may do serious damage to Man's interest 

 through the washing away of fertile top soils, a problem to be 

 considered in a later section. Therefore special provision 

 appears to have been made for encouragement of absorption 

 and infiltration by the relations established between vegeta- 

 tive cover and these functions. 



4 Wells and Subsoil Waters, Bulletin No. 92, Bureau of Soils, U. S. Department of 

 Agriculture, Mar. 26, 1913, p. 8. 



6 The Field, Scope and Status of the Science of Hydrology, in Transactions, American 

 Geophysical Union, Twelfth Annual Meeting, Apr. 30-May 1, 1931, p. 199. 



6 Unpublished manuscript, Interrelations of Ground-Water, Rainfall, Run-off, Floods and 

 Soil Erosion, with Special Reference to the Upper Mississippi Valley, March 1934. 



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